right in your building.
Daily, the custodians collect empty rolls from paper towels. . .and these are the really awesome, thick ones that can lend themselves to all kinds of projects. Give them a plastic bin and ask them to save these for you.
Similarly, the packaging for produce often presents itself as a great resource for projects children can make. Look around. . .involve the other workers in the building in order to maximize resources and cut down on waste. It inspires creativity everywhere!